Method for reducing leakage currents of active area diodes and source/drain diffusions

ABSTRACT

A fabrication method for providing isolation between adjacent regions of an integrated circuit includes providing a guard layer over field edges that are the interfaces between field oxide regions and diffusion regions in which dopant is introduced. The guard layer will inhibit introduction of dopant along the field-edge, so that a substantially dopant-free transition strip is formed. The transition strip inhibits current leakage from the active region to the field oxide region. In one embodiment, the active region is an active area diode, such as used to form an Active Pixel Sensor (APS) pixel. The guard layer is biased so as to further inhibit current leakage during circuit operation. In another embodiment, the method is used in the fabrication of transistors for APS pixels having an overlay photodiode structure.

TECHNICAL FIELD

The invention relates generally to methods for providing isolationbetween adjacent regions of an integrated circuit and more particularlyto methods of reducing current leakage from an active region to a fieldoxide region in a circuit, such as an image sensor circuit.

BACKGROUND ART

Complementary metal-oxide-semiconductor (CMOS) technology is used in thedesign and fabrication of integrated circuits for many types ofapplications. CMOS technology uses n-type transistors (NMOS) and p-typetransistors (PMOS) that are formed by doping selected regions of asubstrate and by forming layers on the substrate. A p-type material,such as boron, may be introduced to a bulk silicon substrate in ablanket ion implantation step. Field oxide regions and n-type regionsmay then be formed using well known integrated circuit fabricationtechniques. Similarly, the processes for depositing conductive anddielectric layers on the substrate to complete the circuit are known.

One general area for applying CMOS technology that has receivedsignificant attention is image capture and processing. Imagingapplications include video, still photography, and navigation that isbased upon optical detection. Linear or two-dimensional arrays of pixelsare formed along the surface of the substrate, with each pixelperiodically generating a signal having a current or voltage level thatis indicative of the intensity of light incident to that pixel. Atypical three-transistor pixel 10 that is used in current CMOS imagesensors is shown in FIG. 1. Sensors that use this technology are oftenreferred to as CMOS active pixel sensors (APS). A timing diagram for theoperation of the three-transistor pixel 10 is shown in FIG. 2. Intypical operation, a node N1 is set to a predetermined voltage V_(dd′)(which may be different than the circuit operating voltage V_(dd)) byturning on an n-channel reset transistor 12. The state of the resettransistor is determined by controlling a reset voltage (V_(reset)). InFIG. 2, V_(reset) goes high at time T0, causing the node N1 to ramp toV_(dd′). At time T1, the reset transistor 12 is turned off andphotoelectrons are generated by the incident light on a photodiode 14.The photoelectrons are injected into node N1, reducing the voltage onthat node by a value of V_(sense=V)_(dd′)−(I_(photo)×T_(illuminate)/C_(N1)). In this equation, I_(photo) isthe photocurrent induced by the incident light, T_(illuminate) is theillumination time period and C_(N1) is the capacitance on node N1. BothV_(dd′) and V_(sense) can in principle be read out of the pixel througha source-follower 16 by activating a row-select transistor 18. In atwo-dimensional array of pixels, there typically are row-selecttransistors and column-select transistors that allow the pixels to besequentially sampled. The row-select transistor 18 is activated bymanipulating a row-select (RS) signal. The illumination on the pixel isthen proportional to V_(dd′)−V_(sense)=I_(photo×T) _(illuminate)/C_(N1,)Persons skilled in the art refer to this operation as Correlated DoubleSampling (CDS). Sampling occurs at time T2 before T_(illuminate) andtime T3 during T_(illuminate.) The pixel is reset at time T4, sinceV_(reset) is caused to go high.

One of the major problems of using CMOS technology in imaging sensors isthe relatively large dark current intrinsic to the CMOS process. Asignificant cause of the large dark current is the reverse-bias diodeleakage in the photodiode 14 of a pixel, as well as in the sourcediffusion of the MOS field effect transistor (MOSFET) 12 connected tothe photodiode. The diode leakage is often dominated by the edge leakagecurrents. Furthermore, in deep-submicron generations of CMOS technology,this leakage current will only increase and take major engineeringefforts to suppress.

The physical layout of the CMOS APS pixel 10 of FIG. 1 will be describedwith reference to FIGS. 3, 4 and 5. FIG. 3 is a top view of the circuitlayout of the APS pixel of FIG. 1, showing the various layers anddiffusion regions. FIG. 4 is a top view that isolates the active areadiode of FIG. 3, while FIG. 5 is a side sectional view of FIG. 4. Theactive area diode is illustrated as being an n+/p diode fabricated in ap-substrate or p-well. However, the descriptions of the operations andproblems apply equally to a p+/n diode in an n-substrate or n-well. Thepn-junction of the diode 14 is defined by the p-substrate or p-well 20,which will be referred to as the p-layer. Electrical connections 22 and24 to the diode are formed by depositing layers that are in contact withan n+ region 26 and a p+ region 28, respectively. The n+ region 26 maybe formed by ion implantation or other doping techniques into the activearea that is identified as the photodiode 14 in FIG. 3. The active areais delineated by a field oxide (FOX) region 30. Typically, the FOXregion is a thick layer of silicon dioxide (SiO₂) that electricallyisolates the active area from other regions of the substrate, which istypically a silicon substrate. There are several well known processesfor forming the FOX. Any of the processes may be used to form the FOX ofFIGS. 3-5. However, each of the known processes is susceptible to theformation of a high density of defects at the edges of the FOX. Thedefects are primarily due to mechanical stress effects andcontamination. The high density of defects located within thepn-junction diode's depletion region contributes to the highreverse-bias leakage current found at the field-edge of the diode. Therehas been much research and development regarding providing process steps(such as oxide deposition, etching and annealing) that minimize the edgeleakage. However, the edge leakage problem is expected to become worseas the CMOS process is applied at the deep-submicron level.

Referring specifically to FIGS. 1 and 3, the gates of the threetransistors 12, 16 and 18 are formed by a patterned polysilicon layer.The polysilicon layer is identified by hatching in FIG. 3. The resettransistor 12 has a gate 32, the row-select transistor 18 has a gate 34,while the transistor 16 has a gate 36 that is electrically coupled tothe N1 node 22. The source/drain regions of the three transistors areformed by diffusions using the appropriate dopants. As can be seen inFIGS. 1 and 3, the transistors 16 and 18 have source/drain regions thatare formed by a common diffusion region 38.

Dark current in the CMOS APS pixel 10 with an active area photodiode 14is caused mainly by the photodiode leakage, which bleeds charge from thenode 22 (N1). This reduces the voltage on the node, even when the resettransistor 12 is turned off during the illumination time T_(illuminate).Therefore, the diode leakage produces an offset in the differentialvoltage produced by the illumination, given by V_(dd′)−(I_(photo)+I_(dark))×T_(illuminate)/C_(N1). For low light illumination, it ispossible for I_(dark) to be approximately the same as I_(photo,) Thus,the dark current limits the dynamic range of the image sensor. Darkcurrent reduction has usually been addressed by attempting to lower theintrinsic diode leakage of the CMOS technology via processing steps.This minimization of the diode leakage characteristics is very difficultin advanced deep-submicron CMOS technologies that use advanced fieldoxide formation techniques and have much higher doping concentrations inthe diode.

Field-edge leakage can also be a significant problem at the transistorlevel of the pixel 10. Each of the three MOSFETs 12, 16 and 18 is formedby growing a thin gate oxide over the active area of the transistor andthen forming the gates 32, 34 and 36 by patterning strips of polysiliconover the thin gate oxide. The n+ dopant is implanted after the gateshave been formed. For each transistor, two separate n+/p diodes are thensimply the source and drain diffusions in combination with the p+contact. Because the source and drain diffusions are delineated by fieldoxides on three sides, they have the same field-edge leakage problem asthe isolated n+/p diode.

There have been attempts to isolate active components of twocomplementary transistors. One such attempt is described in U.S. Pat.No. 5,847,433 to Kerber, which is not related to imaging applications.In order to produce isolated active regions of a CMOS circuit, a fieldplate is formed and doped jointly with wells located between the fieldplate. Therefore, the field plate includes an n-doped region and ap-doped region, as well as a boundary region between the two dopedregions. Each doped region is electrically connected to the well that islocated beneath it. As a result, a flat band condition prevails at thesubstrate surface. While the Kerber method provides an improvement, darkcurrent leakage is still a problem, particularly in imagingapplications.

What is needed is an integrated circuit fabrication method that isapplicable to imaging applications and that provides further reductionsin the edge leakage component of diodes and source/drain diffusions ofMOSFETs.

SUMMARY OF THE INVENTION

A fabrication method for providing isolation between regions of anintegrated circuit includes forming a guard layer on a peripheralportion of at least one of two adjacent regions of a substrate, so thatwhen dopant is introduced into the substrate, an intermediate transitionstrip is left substantially dopant-free. The transition strip inhibitscurrent leakage from an active region to an adjacent region, which ispreferably a field oxide region. In the preferred embodiment, the activeregion is an active area diode, such as the one used in an Active PixelSensor (APS) pixel. Also in the preferred embodiment, the guard layer isa conductive material that is connected to a source of a bias voltage.For an APS pixel, the biased guard layer blocks the doping of the activearea diode during fabrication and effectively blocks edge leakagecurrent during circuit operation, as long as the guard layer is biasedbelow the threshold voltage of the MOS system in which it is formed. Theuse of some process technologies will require bias in accumulation.

In one embodiment, the guard layer is formed of a patterned conductivematerial (such as polysilicon) on the peripheral portion of the activeregion. The guard layer extends across the peripheral portion to atleast the edge of the adjacent field oxide region. Optionally, the guardlayer extends onto a peripheral portion of the field oxide region,thereby relaxing the alignment tolerances. The overlap of the guardlayer onto the active area will depend upon factors such as thealignment specifications of the photolithography tools used in thefabrication technology, but will typically be about the same as theoverlap of the guard layer onto the field oxide region, if such anoverlap is utilized. Since depositing polysilicon is a standard part ofa CMOS circuit, such a modification of the existing active area diode isrelatively simple to accomplish.

In an alternative application, the guard layer is patterned conductivematerial that extends along the interface between a source/drain regionof a MOSFET and a field oxide region. The dopant-free transition stripthat is formed by using the guard layer as a shield in a subsequent stepof introducing dopant into the source/drain region isolates the dopedregion from the field oxide region. In this embodiment, the guard layeris preferably formed of polysilicon and is simultaneously formed withand connected to the gate of the MOSFET. When the gate is biased to ahigh voltage (i.e., the MOSFET is turned on), there will be an inversionlayer generated under the guard layer, thereby extending along theinterface of the source/drain region and the field oxide edge.Optionally, the guard layer may be patterned about both of thesource/drain regions. Connecting the polysilicon guard layer to the gateof the MOSFET has advantages with regard to fabrication simplicity.However, the practice may result in an unacceptable leakage level (dueto the gate-edge leakage) and/or an unacceptable increase insource/drain capacitance. In addition, because of the standard overlapof the gate region onto the source/drain regions, gate-to-source/draincapacitance will increase. If the extra leakage or capacitance isproblematic, it is possible to separate the gate polysilicon from thepolysilicon guard layer. In this case, the polysilicon guard layershould be biased by another contact.

In a third embodiment, the guard layer is used for MOSFETs that sharediffusions. MOSFETs that share a common diffusion are often used in CMOScircuits to lower the total layout area. By depositing the guard layeralong the interface between an adjacent field oxide region and a commondiffusion region, edge leakage is reduced. In this embodiment, the guardlayer may be connected to one of the gates of the two MOSFETs, but notboth, unless the two gates are tied to the same circuit node. Forapplications in which the two transistors are operated separately, theguard layer may be connected to one of the two gates or may be connectedto a separate contact.

An advantage of the invention is that depositing the guard layer priorto introducing the dopant into an active region provides control overedge leakage without significantly diverging from existing fabricationtechniques. In the preferred embodiment, the guard layer is formed ofpolysilicon that is formed at the same time as the gates of thetransistors. The guard layer has the same thickness as the gates andpreferably has the same width, but this is not critical.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a three-transistor CMOS Active Pixel Sensor(APS) pixel for use in imaging applications.

FIG. 2 is a timing diagram for the three signals controlled in theoperation of the APS pixel of FIG. 1.

FIG. 3 is a top view of a prior art circuit layout of layers anddiffusion regions for forming the APS pixel of FIG. 1.

FIG. 4 is a top view of a simplification of the photodiode of the APSpixel of FIGS. 1 and 3.

FIG. 5 is a side sectional view of the photodiode of FIG. 4, taken alonglines 5—5.

FIG. 6 is a top view of a circuit layout of layers and diffusion regionsfor forming the APS pixel of FIG. 1, but with a leakage-control guardlayer in accordance with one embodiment of the invention.

FIG. 7 is a top view of the photodiode of FIG. 6.

FIG. 8 is a side sectional view of the photodiode of FIG. 7, taken alonglines 8—8.

FIG. 9 is a top view of a simplification of a transistor in accordancewith the prior art.

FIG. 10 is a side sectional view of the prior art transistor of FIG. 9,taken along lines 10—10.

FIG. 11 is a top view of the transistor of FIG. 9, but with a guardlayer formed in accordance with an embodiment of the invention.

FIG. 12 is a side sectional view of the transistor of FIG. 11, takenalong lines 12—12.

FIG. 13 is a top view of the transistor of FIG. 9, but with a guardlayer in accordance with another embodiment of the invention.

FIG. 14 is a top view of a simplification of adjacent MOSFETs that sharea common diffusion in accordance with the prior art.

FIG. 15 is a top view of one embodiment of forming a guard region alongthe common diffusion of FIG. 14 in accordance with the invention.

FIG. 16 is a top view of another embodiment of forming a guard layer forthe common diffusion of FIG. 14 in accordance with the invention.

FIG. 17 is a top view of a circuit layout of layers and diffusionregions for forming an overlay photodiode in accordance with the priorart.

FIG. 18 is a side sectional view of the prior art device of FIG. 17,taken along lines 18—18.

FIG. 19 is a top view of a circuit layout of the layers and diffusionregions of FIG. 17, but with a guard layer in accordance with theinvention.

FIG. 20 is a side sectional view of the device of FIG. 19, taken alonglines 20—20.

FIG. 21 is a process flow of steps for forming the circuits inaccordance with the invention.

DETAILED DESCRIPTION

As previously noted, one of the major problems of using CMOS technologyin imaging sensors, such as those used in video, still photography andoptical navigation, is that the CMOS process has a relatively largeinherent dark current. One of the major causes of the large dark currentis the reverse-bias diode leakage in the photodiode. This diode leakageis dominated in many cases by edge leakage currents. The invention thatwill be described with reference to FIGS. 6-8 will address this cause.Another cause is the reverse-bias diode leakage in the source/draindiffusions of MOSFETs connected to the photodiode. The means ofaddressing this cause of the large dark current will then follow.

With reference to FIG. 6, the circuit layout that is shown is consistentwith the circuit layout of FIG. 3. However, a guard layer 40 (shown asbeing hatched) has been incorporated into the fabrication of thephotodiode 14. The guard layer is preferably formed of polysilicon atthe same time that the three polysilicon gates 32, 34 and 36 are formed.Since the incorporation of the guard layer does not affect the schematicrepresentation of the electrical circuit, the reference numerals thatwere used in FIGS. 1 and 3 are also used in FIG. 6. The polysiliconguard layer 40 is formed before the dopant is introduced into the activearea of the photodiode. Therefore, the polysilicon guard layer willblock the n+ implant, leaving a transition strip below the portion ofthe guard layer that overlaps the region surrounded by the field oxide30.

FIGS. 7 and 8 correspond to FIGS. 4 and 5, but include the polysiliconguard layer 40. The transition strip 42 that is substantially dopantfree can be seen in FIG. 8. The polysilicon guard layer 40 isillustrated as being deposited on a portion of the gate oxide layer thatis used in forming the gates of the three transistors. This providesfurther compatibility between the conventional steps of forming thetransistor gates and the steps for forming the guard layer. However,other approaches may be utilized. For example, if the guard layer 40 isto be allowed to have a “floating” potential, rather than being biased,the portion of the gate oxide layer 41 below the guard layer 40 may beeliminated. This would short the guard layer to the n+ region 26.

As seen in FIG. 6, the guard layer 40 connects to the gate 32 of thereset transistor 12. During the illumination period, the gate of thereset transistor is at  volts (as indicated in the timing diagram ofFIG. 2). The bias of the guard layer provides a reduction in darkcurrent. For CMOS processes that do not exhibit the expected leakagecurrent reduction at  volts, it is possible to bias the guard layer 40at a lower voltage, so as to bring the underlying silicon intoaccumulation. This may be performed by separating the guard layer fromthe gate 32 and providing a separate electrical connection to a fixedsource of a negative voltage. Alternatively, the reset transistor 12 maybe driven to a negative gate voltage, rather than a  voltage. In eithercase, the simple circuit layout technique mitigates the need ofextensive and expensive technology development to ensure low darkcurrent in CMOS APS pixels 10.

During fabrication, the polysilicon guard layer 40 blocks theimplantation of dopant, as indicated by the transition strip 42 in FIG.8. During operation, the guard layer is biased below the thresholdvoltage of the MOS system in which it is formed. Consequently, the n+region 26 will be electrically and structurally isolated from the fieldoxide edge 30. It follows that the edge component of diode leakage willbe reduced. How much of a reduction of leakage current that is obtainedwill depend on the exact processing steps used to fabricate the diode14. This is because at a typical guard layer bias of  volts, there is adepletion region under the polysilicon that can still allow leakagecurrent as a result of field-edge defects. However, as noted above, ifleakage currents are not sufficiently reduced, it is possible to biasthe polysilicon so as to bring the underlying substrate region intoaccumulation. In this case, there should be no contribution to leakagefrom the field-edge defects.

The overlap of the guard layer onto the active area diode 14 will dependupon the alignment specifications of the photolithography tools used inthe technology. Typically, the overlap will be approximately the same asthe overlap length of the guard layer onto the field oxide region 30.

A comparison of FIGS. 5 and 8 shows that the use of the guard layer 40reduces the area of the n+ region 26. Consequently, the sensitivity tolight will be reduced. If one wants to retain the same active diodearea, the structure of the diode can be increased in size.

While the guard layer has been described as being formed of polysilicon,this is not critical. Other materials may be substituted. Preferably,the material is conductive, so that it can be biased below the thresholdvoltage of the MOS system. However, the use of a dielectric guard layerprovides some of the advantages of the polysilicon guard layer, if itblocks the dopant implantation during the fabrication process. Also inthe preferred embodiment, the guard layer is not removed after thedopant implantation step, but applications in which the guard layer isremoved have been contemplated.

Similar to the active area diode, the MOSFET diffusions that form thesource/drain regions of the transistors can benefit from the use of aguard layer over the field-edge that forms their boundaries. Theconventional MOSFET transistor is shown in FIGS. 9 and 10. A MOSFET 44is formed by growing a thin gate oxide 46 over the active area of thetransistor. A layer of polysilicon is then formed over the gate oxide 46by material deposition and pattern etching. This provides the gate 48 ofthe MOSFET. The source/drain regions 50 and 52 are then formed byimplantation of an n+ dopant that extends to a field oxide region 54.Contacts 56 and 58 may be patterned to reside on the source/drainregions. A p+ layer 60 is also shown in FIGS. 9 and 10.

As a result of the fabrication process, two separate n+/p diodes 62 and64 are formed, with the source/drain regions providing the n+ contactand the substrate 66 providing the p contact. Because the source anddrain diffusions are delineated on three sides by the FOX 54, they havethe same field-edge leakage problem as the active area diode describedabove. Referring now to FIGS. 11 and 12, the formation of the gate oxide46 and the polysilicon that forms the gate 48 can be extended to providea guard layer 68 around the drain region 52 of the transistor 44. Whilethe illustration shows the guard layer as being applied only to thedrain region, it is possible to achieve the same benefits for the sourceregion 50 if the guard layer is patterned over the edge of the sourceregion along the FOX 54. Optionally, the guard layer can be formed aboutboth of the source/drain regions.

By forming the guard layer 68 prior to introducing the dopant into thesource/drain regions 50 and 52, a transition strip 70 that issubstantially free of dopant is formed at the edge of the FOX 54. Thisprovides the same advantages described with reference to FIGS. 4-8.Specifically, the edge component of the diode leakage is significantlyreduced. However, when the guard layer 68 is connected to the gate 48 inthe manner shown in FIG. 11, the bias of the gate to a high voltage (FETturned on) will create an inversion layer below the entire polysiliconto extend the n+ region 26 to the FOX 54. The result may be an increasedleakage (due to gate-edge leakage) and extra source/drain capacitance.In addition, because of the standard overlap of the gate onto thesource/drain regions, gate-to-source/drain capacitance is increased. Ifthe extra leakage or capacitance is problematic, it is possible toseparate the gate polysilicon 48 from the polysilicon material thatforms the guard layer. This embodiment is shown in FIG. 13. Preferably,the guard layer 72 of FIG. 13 is biased by connection to a fixed sourceof voltage (e.g.,  volts). The drain region 52 will have small links 74and 76 in which the diffusion region contacts the FOX 54, so as tocontribute to leakage current. The dimensions of the links 74 and 76will depend upon the spacing rule of the technology's polysilicon. Theextra leakage will be negligible for large MOSFETs.

The invention is also applicable to MOSFETs that share diffusions. Theconventional structure is shown in FIG. 14. A pair of MOSFETs are shownin a side-by-side arrangement. The first transistor includes apolysilicon gate 78 and a source region 80, while the second transistorincludes a polysilicon gate 82 and a diffused source region 84. The twotransistors share a drain 86 diffusion. MOSFETs that share a commondiffusion are often used in CMOS circuits to lower the total layoutarea.

Referring now to FIG. 15, the common drain 86 has reduced leakage as aresult of a polysilicon guard 88 that is formed on opposite sides of thecommon drain. In the embodiment of FIG. 15, the guard 88 is connected tothe second gate 82 in order to provide a bias. The guard is separatedfrom the first gate 78, so that the two gates are not shorted. Ofcourse, if the gates are tied to a single circuit node, the polysiliconguard 88 can be connected to both gates. This gate-to-gate coupling isparticularly useful for MOSFETs with multiple gate fingers.

FIG. 16 shows a modification of the embodiment of FIG. 15. In thisembodiment, the guard layer 90 is electrically separated from both ofthe gates 78 and 82. As previously noted, this approach may be used toreduce drain capacitance. The guard 90 of FIG. 16 is preferablyconnected to a source of a fixed voltage, such as electrical ground.

There are advantages to implementing a CMOS APS pixel 10 of the typeshown in FIG. 1 using a photodiode 14 that overlays the underlying CMOSchip. One advantage is that the bulk photodiode is removed from thesubstrate, so that the only diode that can contribute to dark current isthe one formed by the source diffusion of the reset transistor, whichhas a much smaller area than the substrate embedded photodiode 14 ofFIG. 3. An exemplary circuit layout of an overlay photodiode arrangementis shown in the top view of FIG. 17 and the side sectional view of FIG.18. In the conventional approach, the reset transistor 12 is formedusing conventional techniques. As a result, a FOX region 92 andsource/drain regions 94 and 96 are formed in a substrate 98. A gateoxide layer 100 and the polysilicon gate 32 are patterned before ionimplantation forms the source/drain regions.

After the transistors 12, 16 and 18 are formed at the substrate level, adielectric layer 102 is deposited at the same level as formation of apair of contacts 104 and 106 to the source/drain regions 94 and 96.Other dielectric layers 108, 110 and 112 are formed, with each level ofdielectric material including a patterned metal 114, 116 and 118, aswell as a patterned contact 120, 122 and 124. The patterned metal layersand contact layers form a via from the transistor 12 to an overlayphotodiode 126. Correlating FIGS. 17 and 18, the portion of the metallayer 114 that resides above the contact 106 (C1) in FIG. 18 is the node22 (N1) in FIG. 17.

Although the source diffusion 96 of the reset transistor 12 isrelatively small, it may still induce a relatively large dark current,especially in advanced deep-submicron CMOS technologies and in fact maybe the largest contributor to dark current in the pixel. The approach ofproviding a polysilicon guard layer may be applied to the overlayphotodiode structure in order to address this possibility. Specifically,a guard layer of polysilicon may be formed over the field-edge of thesource diffusion in order to reduce dark current. Because the sourcediffusion 96 is a small portion of the total pixel area, adding theguard layer will result in only a small increase in pixel area. FIGS. 19and 20 illustrate the overlay photodiode with the diffusion-inhibitingguard layer 128 about the source diffusion 96. The guard layer is formedbefore the dopant is introduced into the source region. Therefore, atransition strip that is relatively free of dopant is formed between theFOX 92 and the source region 96. The transition strip 130 is illustratedin FIG. 20.

The steps for carrying out the invention will be described briefly withreference to FIG. 21. In step 132, a circuit layout is selected. In thepreferred embodiment, the circuit layout defines layers of a CMOSimaging sensor having an array of APS pixels. The APS pixel 10 of FIG. 1is merely one example. The array of pixels may be linear or may betwo-dimensional.

In step 134, the field oxide regions are formed. Conventional techniquesmay be used to form these regions. The approach is not critical to theinvention.

A diffusion-inhibiting guard layer is formed over selected field edges,as shown at step 136. Some of the alternatives were described andillustrated above. Preferably, the guard layer is formed of polysiliconand is formed simultaneously with the gates of transistors.

The dopant is introduced into the appropriate regions at step 138. Theregions that are guarded by the guard layer that was formed in step 136may be active area diode regions, source/drain diffusion regions, orboth. The remainder of the circuit fabrication is then completed at step140. The guard layer or guard layers are biased at step 142 in order toinhibit edge leakage.

What is claimed is:
 1. A method of forming an active area diode for animaging circuit comprising steps of: forming a field oxide region on asubstrate, said field oxide region having an edge that at leastpartially defines a geometry of an active area diode region; forming aconductive layer on said substrate such that said conductive layergenerally follows a contour of said edge of said field oxide region,while extending onto a peripheral portion of said active area dioderegion; and modifying electrical characteristics of said active areadiode region such that said active area diode region is responsive tolight energy, said conductive layer inhibiting said modification of saidelectrical characteristics within said peripheral region.
 2. The methodof claim 1 further comprising a step of connecting said conductive layerto a constant source of voltage.
 3. The method of claim 1 wherein saidstep of modifying said electrical characteristics includes diffusing adopant into said active area diode region.
 4. The method of claim 1wherein said step of forming said conductive layer includes patterningpolysilicon to extend onto peripheral portions of each of said fieldoxide region and said active area diode region.